I think the root cause is in the tools.

github.com/golang/tools/go/analysis/passes/structtag

	for _, enc := range [...]string{"json", "xml"} {
		switch reflect.StructTag(tag).Get(enc) {
		// Ignore warning if the field not exported and the tag is marked as
		// ignored.
		case "", "-":
		default:
			pass.Reportf(field.Pos(), "struct field %s has %s tag but is not exported", field.Name(), enc)
			return
		}
	}

Change https://go.dev/cl/387114 mentions this issue: go/analysis/passes/structtag: ignore "-" fields

My understanding is that the vet check is WAI.

c int `json:"-,omitempty"`

is naming the unexported field of X c as "-" within the json and adding the "omitempty" option to it. The same is then done with d. This also repeats a json name "-".

This is my reading of https://pkg.go.dev/encoding/json:

The encoding of each struct field can be customized by the format string stored under the "json" key in the struct field's tag. The format string gives the name of the field, possibly followed by a comma-separated list of options. The name may be empty in order to specify options without overriding the default field name.

and

As a special case, if the field tag is "-", the field is always omitted. Note that a field with name "-" can still be generated using the tag "-,".

Someone authoritative on "encoding/json" want to weight in?

If this is WAI, we might want to add additional checks so this case is easier to understand. Open to suggestions about how. (Complain about using "-" on "a" and "b" as they are unexported?) I think new checks would need a proposal.

cc owner @rsc https://dev.golang.org/owners

is naming the unexported field of X c as "-" within the json and adding the "omitempty" option to it. The same is then done with d. This also repeats a json name "-".

Second this. "c" is interpreted as a json field with name "-" and an extra tag "omitempty"

  c int `json:"-,omitempty"`

In addition, using json:"-,omitempty" seems to be improper since "-" indicates to skip the field and "omitempty" indicates to skip the field when it is empty. It suffices to have only the "-". So this warning correctly reports something that shall be rectified.

I agree with @timothy-king,
"-" is an valid field name according to the document which we should do a duplicate checks on.

https://pkg.go.dev/encoding/json@go1.17.8

// Field appears in JSON as key "-".
Field int `json:"-,"``

Is it ok to close this issue?

Case split in my mind:

1. If we think this is WAI, closing this issue SGTM.
2. If we think we want better diagnostics but not new ones, we can keep this open and close after we improve them.
3. If we think we want new diagnostics, I would close this and open a proposal.

Line between "better" and "new" is fuzzy. But I would put warn on "-" on any exported field as "new" while having a special case for when a duplicate json tag is named "-" would be "better". Such as struct field d repeats json tag "-" also at main.go:6 (Note: json:"-," renames d to "-").

Open to all three choices though.

Looks like the consensus is working as intended